Abbie's poem describes the moment a person realizes someone has come up behind them without turning around to look. The speaker feels it first in the air, which seems to go still, as if the breeze has stopped on purpose. Then the body reacts: the hair on the back stands up, the heart speeds up, and even the ground seems to shake. The comparison to a barometer is a nice touch, since a barometer picks up a change in pressure before the weather arrives. In the same way, the speaker feels the atmosphere "getting thicker" before anything is seen.
The last two lines shift from description to questions. "That particular person entered the room, didn't he?" suggests the speaker already knows who it is, and the phrase "I know this feeling" hints at a familiar, perhaps uneasy presence. The mood is tense and suspenseful, and ending on questions leaves us waiting, just as the speaker is, for what happens next.